Screen printing methods have been used for printing a pattern of cream solder or the like on a circuit board in an electronic component mounting process. Circuit boards have become more and more minute in recent years due to miniaturization of electronic devices, because of which the demand for highly precise printing of materials such as cream solder has been increased.
A screen printing apparatus employing a conventional screen printing method is explained by referring to FIGS. 5 and 6.
Referring to FIG. 5, the screen printing apparatus comprises a circuit board recognition unit A and a printing unit B. Reference numerals 1, 2, and 3 respectively represent a circuit board, board recognition marks provided on the circuit board 1, and a stage for positioning the circuit board 1, which is driven by an X-axis motor 9 for traveling along the X direction between the circuit board recognition unit A and the printing unit B.
The printing unit B includes a screen plate 4, screen recognition marks 5, 5 provided on the screen plate 4, a camera 6 for visually recognizing the board recognition marks 2, 2 and the screen recognition marks 5, 5, a screen frame 7 with which the screen plate 4 is secured, a screen holder 8 which holds the screen frame 7, a Y-axis motor 10 for driving the stage 3 along the Y direction, a .theta.-axis motor 11 for driving the stage 3 along the .theta. direction, a printing paste 12, and a left squeeze 13 and a right squeeze 14 which run directly on the screen plate 4 along the horizontal direction to print the printing paste 12 on the circuit board 1.
Actions according to the conventional screen printing method is explained referring to the flowchart of FIG. 6.
At Step #21 shown in FIG. 6, a teaching operation is performed in which the two screen recognition marks 5, 5 on the screen plate 4 are recognized by the camera 6, and the distances (SX1, SY1) and (SX2, SY2) respectively from the camera origin point are recorded. Also, the board recognition marks 2, 2 on the circuit board 1 are recognized by the camera 6, and the distances (PX1, PY1) and (PX2, PY2) respectively from the camera origin point are recorded, before the procedure moves to Step #22.
At Step #22, a circuit board 1 is loaded into the circuit board recognition unit A and positioned on the stage 3, and the procedure goes to Step #23.
At Step #23, the two board recognition marks 2, 2 on the circuit board 1 are recognized by the camera 6, and the amount of displacement from the points (PX1, PY1) and (PX2, PY2) of the board recognition marks 2, 2 taught at Step #21 is calculated so as to determine the amount of movement of the stage 3 in the X, Y, and .theta. directions by the X-axis motor 9, Y-axis motor 10, and .theta.-axis motor 11 which are all connected to the stage 3.
This is followed by Step #24 where the stage 3 is driven to move by the X-axis motor 9, Y-axis motor 10, and .theta.-axis motor 11 according to the determined amount of movements, after which the stage 3 is transferred from the circuit board recognition unit A to the printing unit B, and the procedure advances to Step #25.
At Steps #25 and #26, the stage 3 is lifted up so as to bring the circuit board 1 into contact with the screen plate 4, and the procedure goes to Step #27.
At Step #27, the two squeezes 13 and 14 are descended to contact the screen plate 4 and moved to the right or left in direct contact with the screen plate 4 to print the printing paste 12 on the circuit board 1, and the procedure goes to Step #28.
At Step #28, the circuit board 1 which has been printed with the paste is unloaded and Step #29 follows.
It is judged at Step #29 whether the printing operation is finished or not. If not, the procedure returns to Step #22. If the printing has been finished, the procedure is terminated.
However, it is not perfectly accomplished that the stage 3 is moved from the circuit board recognition unit A to the printing unit B with high accuracy along the X and Y directions horizontally and lifted vertically after the circuit board 1 on the stage 3 is precisely positioned in the circuit board recognition unit A, hence causing discrepancy in locations.
Also, since a circuit board 1 of high mounting density in recent years carries very minute or narrowly pitched patterns, it is hardly possible to make all the patterns of lands on the circuit board 1 to correspond to all the patterns of apertures in the screen plate 4, however strictly the accuracy of dimensions of both the circuit board 1 and the screen plate 4 is controlled. Even when both recognition marks are in register with each other a, very small discrepancy is inevitable in a part containing minute or narrowly pitched patterns, which may cause an imperfect print such as a bridge. A common method for correcting such a discrepancy in positions of patterns comprises the steps of printing paste 12 on the circuit board 1 on trial, measuring either optically or with the use of an instrument the amount of discrepancy in positions of the land pattern and the pattern of paste 12 printed in a portion where minute or narrowly pitched patterns are formed, saving the amount measured as a data for correction, and adding this amount for correction to the amount of displacement calculated at the Step #23 described above in each printing process. In this case, since the squeezes 13, 14 move to the right and left respectively, at least two trial printing actions with the squeezes 13, 14 in two directions, leftward and rightward, are needed, wherefore it takes time when the type of circuit board to be manufactured is changed from one to another.
It is an object of the present invention to provide a screen printing method and an apparatus therefor, by which the above problems are solved, i.e., the work time for changing the type of circuit board is shortened since no test printing is required, as well as minute or narrowly pitched patterns are quite accurately screen printed.